1. Field of the Invention
The present invention generally relates to fracture and wear resistant cutting elements for down hole cutting tools. More specifically, the invention relates to composite materials for cutting elements used on down hole cutting tools, such as rock bits, which enhance the useful life of the tools incorporating the same.
2. Background Art
Drill bits used to drill wellbores through earth formations generally can be categorized within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements are typically referred to as “shear cutters” and may be formed from any one or combination of hard or superhard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits may include “inserts” as cutting elements, which are press fit (i.e., interference fit) into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Breakage or wear of cutting elements, among other factors, limits the longevity of a drill bit. For example, cutting elements used with a rock bit are generally subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impact loads from contact with a borehole bottom. The high wear loads can also cause thermal fatigue in the cutting elements, which initiates surface cracks on the cutting elements. These cracks are further propagated by a mechanical fatigue mechanism that is caused by the cyclical bending stresses and/or impact loads applied to the cutting elements. Fatigue cracks may result in chipping, breakage and failure of cutting elements.
Cutting elements, such as gage inserts on a roller cone bit which primarily function to cut the corner of a borehole bottom are subject to a significant amount of thermal fatigue. This thermal fatigue is caused by heat generated on the gage side of an insert by friction when the insert engages the borehole wall and slides into a bottom-most crushing position. When the insert rotates away from the bottom, it is quickly cooled by the surrounding circulating fluid. Repetitive heating and cooling of the insert initiates cracking on the outer surface of the insert. Thermal fatigue cracks then propagate through the body of the insert when the crest of the insert contacts the borehole bottom because of the high contact stresses. The time required to progress from heat checking, to chipping, and eventually to broken inserts depends upon the insert material, formation type, rotational speed of their bit, and applied weight on bit, among other factors.
In the case of roller cone bits, even inserts on interior rows are also subject to thermal fatigue caused by scraping the borehole bottom. The amount of scraping varies from row to row and is influenced by bit offset and cone to bit speed ratio, among other factors.
In the case of fixed cutter bits, the shear cutters typically have a body (or substrate), which has a contact face. An ultra hard layer is typically bonded to the contact face of the body by a sintering process to form a cutting face (sometimes referred to as a “cutting table”). The body is typically made of tungsten carbide-cobalt (sometimes referred to simply as “tungsten carbide” or “carbide”). The ultrahard material layer is typically polycrystalline ultrahard material, such as polycrystalline diamond (“PCD”) or polycrystalline cubic boron nitride (“PCBN”). Typically, shear cutters are mounted into a fixed cutter bit body at a negative rake angle. Consequently, the region of the cutting element that makes contact with the earthen formation includes a portion of the ultrahard material layer's upper surface circumferential edge. This portion of the layer is subjected to the highest impact loads and thermal stresses which can result in cracks initiated at the ultrahard material layer. These cracks can propagate into the substrate of the shear cutter. Accordingly, the toughness of the substrate plays a significant role on the brakeage resistance of cutting elements for fixed cutter bits.
Cemented tungsten carbide generally refers to tungsten carbide (WC) particles dispersed in a binder metal matrix, such as iron, nickel, or cobalt. Tungsten carbide in a cobalt matrix is the most common form of cemented tungsten carbide, which is further classified by grades based on the grain size of WC and the cobalt content.
Tungsten carbide cutting elements (inserts or cutters) are primarily made in consideration of two factors that relate to the lifetime of a cutting element: wear resistance and toughness. As a result, existing inserts and shear cutters are generally formed of cemented tungsten carbide particles with average grain sizes of less than 3 μm (micrometers) as measured by ASTM E-112 method and cobalt contents in the range of 6-16% by weight of cobalt. Resulting cutting elements typically have a hardness in the range of about 86 Ra to 89 Ra.
For tungsten carbide/cobalt (WC/Co) systems, it is typically observed that wear resistance increases and fracture toughness decreases as the grain size of tungsten carbide or the cobalt content decreases. On the other hand, fracture toughness generally increases and wear resistance decreases with larger grains of tungsten carbide and/or greater percentages of cobalt. Thus, fracture toughness and wear resistance (i.e., hardness) tend to be inversely related. That is, as the grain size or the cobalt content is decreased, wear resistance of a specimen is improved, and its fracture toughness decrease, and vice versa.
Due to this inverse relationship between fracture toughness and wear resistance, the grain size of tungsten carbide and the cobalt content can be selected to obtain a desired wear resistance or a desired toughness. For example, a higher cobalt content or a larger WC grains may be used when a higher toughness is required, whereas a lower cobalt content and smaller WC grains are used when a better wear resistance is desired. To achieve a desired balance between wear resistance and toughness, conventionally, grain sizes used for cutting elements have remained within the range of about 1 to 3 μm (as measured by ASTM E-112 method). That is, until recently as disclosed in U.S. application Ser. No. 10/017,404 filed Dec. 14, 2001 and U.S. application Ser. No. 10/396,261 filed Mar. 25, 2003, both titled “Fracture and Wear Resistant Rock Bits,” and U.S. application Ser. No. 10/437,750, filed May 14, 2003 and titled “Coarse Carbide Substrate Cutting Elements and Methods of Forming the Same”, all incorporated herein reference. As stated therein, there exists a desire and need for improving the toughness of materials used for cutting elements without significantly reducing the wear resistance and, in some cases, thermal conductivity of the resulting cutting element.